The present invention relates generally to the field of cryogenic air separation and has particular reference to the production of liquid oxygen (xe2x80x9cLOXxe2x80x9d) and the enhanced recovery of krypton and xenon.
Krypton and xenon are present in air at very low concentrations, typically about 1.14 parts per million (xe2x80x9cppmxe2x80x9d) and about 0.087 ppm respectively. They are both valuable gases and, thus, there is an economic incentive to maximise their recovery in an air separation process.
In typical air separation processes, krypton and xenon concentrate in the LOX product taken from the bottom of the low pressure distillation column (xe2x80x9cLP columnxe2x80x9d) as they are far less volatile than oxygen. The smaller the LOX flow, therefore, the more concentrated the krypton and xenon in this product.
In air separation processes in which most of the oxygen product is removed from the LP column in the gas phase, it is possible to make sure that very little krypton and xenon is lost in the gaseous oxygen (xe2x80x9cGOXxe2x80x9d) by removing the GOX several distillation stages above the bottom of the LP column. Almost all of the krypton and xenon entering the air separation plant can then be recovered in the LOX product, which is a very small proportion of the total oxygen flow. This LOX product can then be processed further to produce a krypton and xenon product.
If the LOX flow from the distillation process is much greater, for example when all the oxygen is withdrawn from the distillation column as LOX, pumped to the required pressure and evaporated in the main heat exchanger, the loss of krypton and xenon is much greater, even when the LOX is taken several stage up the LP column, separately from the krypton- and xenon-concentrated liquid stream. Essentially all of the krypton and xenon entering the air separation plant flows down the LP column to the sump of the LP column in the descending liquid, so any liquid withdrawal will remove a portion of the krypton and xenon proportional to the total liquid withdrawn as product. This will typically lead to losses of about 30% of these valuable products.
It is desirable, therefore, to increase the recovery of krypton and xenon from an air separation plant in which at least part of the oxygen product is withdrawn as LOX.
U.S. Pat. No. 5,425,241 (Agrawal et al; published on Jun. 20, 1995) discloses a cryogenic air separation process in which an ultra-high purity oxygen product is produced in an auxiliary stripping column. A first oxygen-containing stream (essentially free of heavier contaminants such as krypton and xenon) is removed from the main distillation column and fed to the top of the auxiliary stripping column. It is stated that this stream can be liquid, vapor or a combination of both. A second oxygen-containing stream (essentially free of lighter contaminants such as nitrogen and argon) is removed from the main distillation column and is used to provide the stripping gas in the auxiliary stripping column. Ultra-high purity oxygen is removed from an intermediate location in the auxiliary stripping column. LOX (having a total contaminant concentration of generally less than 5%) is removed from the LP column. The fate of this product is not disclosed.
GB-A-2346205 (Rathbone; published on Aug. 2, 2000) discloses the production of a krypton/xenon-enriched LOX stream and a high purity LOX stream from a cryogenic air separation process co-producing an argon product. Air is separated in a double column distillation system comprising a high pressure distillation column (xe2x80x9cHP columnxe2x80x9d) thermally integrated with an LP column. It is disclosed that a krypton- and xenon-containing LOX stream is withdrawn from the LP column and may be passed to a storage vessel and taken as desired from the storage vessel for further purification by conventional means so as to produce relatively pure krypton and xenon products. It is also disclosed that an argon-oxygen (but krypton- and xenon-lean) vapor stream is taken from the LP column and separated in a further rectification column into an argon-vapor fraction and an argon-enriched liquid fraction. The argon-enriched liquid fraction is fed to an argon-stripping column to produce a relatively pure LOX fraction. In this process, argon must be separated to be able to achieve a high recovery of krypton and xenon. Furthermore, the teaching of GB-A-2346205 requires that the rare gas lean stream withdrawn from the LP column be a vapour lean in nitrogen and that there be two additional rectification steps in order to obtain a purified oxygen product lean in krypton-xenon. These restrictions add to the cost of obtaining the said purified oxygen stream.
It is desirable, therefore, to provide an air separation process that is able to produce a krypton- and xenon-enriched LOX product for further processing and a pure LOX product without the capital expense and running costs of an argon stripping column.
According to a first aspect of the present invention, there is provided a process for the production of liquid oxygen (xe2x80x9cLOXxe2x80x9d) product and a krypton- and xenon-enriched liquid product from the cryogenic separation of air using a cryogenic distillation system comprising a main distillation system and at least a first additional distillation column, said process comprising:
separating feed air in said main distillation system into nitrogen-rich overhead vapor and said krypton- and xenon-enriched liquid product;
removing at least a portion of said krypton- and xenon-enriched liquid product from said main distillation system for further processing to produce at least one krypton- and/or xenon-rich product;
feeding xenon-lean liquid removed from or derived from liquid removed from said main distillation system to said first additional distillation column; and
separating said portion of xenon-lean liquid in said first additional distillation column into oxygen-rich overhead vapor and said LOX product having a concentration of xenon less than that in said feed air.
The expression xe2x80x9ckrypton- and xenon-enriched productxe2x80x9d is intended to mean that the product has concentrations of krypton and xenon that are greater than their respective concentrations in air. In addition, the expression xe2x80x9cxenon-lean liquidxe2x80x9d is intended to mean that the liquid has a xenon concentration of less than that in air. The xenon-lean liquid is usually also lean in krypton, i.e. has a krypton concentration of less that that in air.
Producing the LOX product from a xenon-lean liquid taken from the main distillation system has the advantage that the LOX product may be produced from the liquid in a single separation step. The present invention does not require the presence of an additional argon separation step producing an argon product to provide a high recovery of krypton and xenon. In addition, it does not require that the xenon-lean liquid be lean in nitrogen also (as required in GB-A-2346205) which allows greater flexibility in selecting the source of the xenon-lean liquid.
Preferably, the process further comprises further processing said krypton- and xenon-enriched liquid product to produce at least one product selected from the group consisting of a krypton-rich product, a xenon-rich product and a krypton- and xenon-rich product. Any known process may be employed for this further processing step such as distillation, adsorption or membrane separation.
In preferred embodiments where the main distillation system comprises at least a high pressure (xe2x80x9cHPxe2x80x9d) distillation column and a low pressure (xe2x80x9cLPxe2x80x9d) distillation column, said columns being thermally integrated via a reboiler/condenser, the process further comprises:
separating feed air in the HP column into nitrogen-enriched overhead vapor and crude liquid oxygen (xe2x80x9cCLOXxe2x80x9d) bottoms liquid;
feeding at least a portion of said CLOX bottoms liquid to said LP column after pressure adjustment;
separating said CLOX bottoms liquid in the LP column into said nitrogen-rich overhead vapor and said krypton- and xenon-enriched liquid product;
condensing at least a portion of said nitrogen-enriched overhead vapor in said reboiler/condenser by indirect heat exchange against krypton- and xenon-enriched liquid product to produce condensed nitrogen-enriched overhead vapor;
feeding at least a portion of said condensed nitrogen-enriched overhead vapor to the HP column as reflux; and
feeding liquid from or derived from the HP column to the LP column as reflux after pressure adjustment.
The xenon-lean liquid may be removed from or derived from liquid removed from the LP column or the HP column.
In embodiments wherein the xenon-lean liquid is derived from liquid removed from the HP column, the cryogenic distillation system may further comprise a second additional distillation column and the process further comprises:
removing xenon-depleted liquid from the HP column and feeding at least a portion of said liquid to said second additional distillation column;
separating xenon-depleted liquid in said second additional distillation column into oxygen-enriched overhead vapor and said xenon-lean liquid;
removing xenon-lean liquid from said second additional distillation column and feeding at least a portion of said liquid, after pressure adjustment, to said first additional distillation column for separation into said oxygen-rich overhead vapor and said LOX product.
The expression xe2x80x9cxenon-depleted liquidxe2x80x9d is intended to mean liquid having a xenon concentration that is less than that in air. The xenon-depleted liquid is usually also depleted in krypton, i.e. has a krypton concentration of less that that in air. The concentrations of krypton and xenon in the xenon-depleted liquid are not necessarily equal to their concentrations in the xenon-lean liquid.
Preferably, the xenon-depleted liquid is substantially free of krypton and xenon.
At least a portion of the oxygen-enriched overhead vapor is usually fed to the HP column.
In embodiments using a second additional distillation column, krypton- and xenon-depleted vapor preferably is fed from the HP column to the second additional distillation column as stripping gas.
In preferred embodiments of the invention, krypton and xenon-lean vapor is fed from the main distillation system to the first additional distillation column as stripping gas. In such embodiments, the first additional distillation column is preferably operated substantially at the pressure of the krypton- and xenon-lean vapor.
Krypton- and xenon-rich vapor may be removed from the main distillation system and fed to the first additional distillation column. The LOX product may then be removed from an intermediate location in the first additional distillation column and bottoms liquid from the first additional distillation column may then be fed to the main distillation system.
A portion of the LOX product may be boiled by indirect heat exchange against a condensing process stream in a reboiler/condenser provided in the first additional distillation column to produce stripping gas.
In preferred embodiments, argon is not separated from said xenon-lean liquid in a separate argon-stripping column. However, an argon-stripping column may be included in the apparatus and the process may then further comprise removing the xenon-lean liquid from liquid feed to the argon-stripping column. Alternatively, a conventional argon column fed by an oxygen-argon vapour from the LP column could be included, if desired.
The concentration of krypton in the xenon-lean liquid is preferably from about 0.0 ppm to about 0.5 ppm and typically about 0.2 ppm. The concentration of xenon in the xenon-lean liquid is preferably from about 0.0 parts per billion (xe2x80x9cppbxe2x80x9d) to about 20 ppb and typically about 10 ppb.
Gaseous oxygen (xe2x80x9cGOXxe2x80x9d) may be removed from the main distillation system as a minor product.
In a preferred process for the production of liquid oxygen (xe2x80x9cLOXxe2x80x9d) product and a krypton- and xenon-enriched liquid product from the cryogenic separation of air using a cryogenic distillation system comprising at least an HP column and an LP column, said columns being thermally integrated via a reboiler/condenser and at least a first additional distillation column, the process comprises:
separating feed air in the HP column into nitrogen-enriched overhead vapor and crude liquid oxygen (xe2x80x9cCLOXxe2x80x9d) bottoms liquid;
feeding at least a portion of said CLOX bottoms liquid to said LP column after pressure adjustment;
separating said CLOX bottoms liquid in the LP column into nitrogen-rich overhead vapor and krypton- and xenon-enriched liquid product;
condensing at least a portion of said nitrogen-enriched overhead vapor in said reboiler/condenser by indirect heat exchange against krypton- and xenon-enriched liquid product to produce condensed nitrogen-enriched overhead vapor;
feeding at least a portion of said condensed nitrogen-enriched overhead vapor to the HP column as reflux; and
feeding liquid from or derived from the HP column to the LP column as reflux after pressure adjustment;
removing at least a portion of said krypton- and xenon-enriched liquid product from said LP column for further processing to produce at least one krypton- and/or xenon-rich product.
removing xenon-lean liquid from said LP column and feeding said liquid to said first additional distillation column; and
separating said portion of xenon-lean liquid in said first additional distillation column into oxygen-rich overhead vapor and said LOX product having a concentration of xenon less than that in said feed air.
In another preferred process for the production of liquid oxygen (xe2x80x9cLOXxe2x80x9d) product and a krypton- and xenon-enriched liquid product from the cryogenic separation of air using a cryogenic distillation system comprising at least an HP column and an LP column, said columns being thermally integrated via a reboiler/condenser and at least a first additional distillation column, the process comprises:
separating feed air in the HP column into nitrogen-enriched overhead vapor and crude liquid oxygen (xe2x80x9cCLOXxe2x80x9d) bottoms liquid;
feeding at least a portion of said CLOX bottoms liquid to said LP column after pressure adjustment;
separating said CLOX bottoms liquid in the LP column into nitrogen-rich overhead vapor and said krypton- and xenon-enriched liquid product;
condensing at least a portion of said nitrogen-enriched overhead vapor in said reboiler/condenser by indirect heat exchange against krypton- and xenon-enriched liquid product to produce condensed nitrogen-enriched overhead vapor;
feeding at least a portion of said condensed nitrogen-enriched overhead vapor to the HP column as reflux; and
feeding liquid from or derived from the HP column to the LP column as reflux after pressure adjustment;
removing at least a portion of said krypton- and xenon-enriched liquid product from said LP column for further processing to produce at least one krypton- and/or xenon-rich product;
feeding xenon-lean liquid removed from or derived from liquid removed from said HP column to said first additional distillation column; and
separating said portion of xenon-lean liquid in said first additional distillation column into oxygen-rich overhead vapor and said LOX product having a concentration of xenon less than that in said feed air.
According to a second aspect of the present invention, there is provided apparatus for the production of LOX product and a krypton- and xenon-enriched product according to the process of the first aspect, said apparatus comprising:
a main distillation system for separating feed air into nitrogen-rich overhead vapor and said krypton- and xenon-enriched liquid product;
a first additional distillation column for separating xenon-lean liquid removed from or derived from liquid removed from said main distillation system into oxygen-rich overhead vapor and said LOX product; and
conduit means for feeding said xenon-lean liquid from said main distillation system to said first additional distillation column.
The main distillation system preferably comprises an HP column and an LP column, said columns being thermally integrated via a reboiler/condenser.
The apparatus may further comprise:
a second additional distillation column for separating xenon-depleted liquid into oxygen-enriched overhead vapor and said xenon-lean liquid;
conduit means for feeding xenon-depleted liquid from the HP column to the second additional distillation column; and
conduit means for feeding xenon-lean liquid from the second additional distillation column to said first additional distillation column.